The Collaborative International Dictionary of English v.0.48:

Plaid \Plaid\, n. [Gael. plaide a blanket or plaid, contr. fr.
   peallaid a sheepskin, fr. peall a skin or hide. CF.
   Pillion.]
   1. A rectangular garment or piece of cloth, usually made of
      the checkered material called tartan, but sometimes of
      plain gray, or gray with black stripes. It is worn by both
      sexes in Scotland.
      [1913 Webster]

   2. Goods of any quality or material of the pattern of a plaid
      or tartan; a checkered cloth or pattern.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Plaid \Plaid\, a.
   Having a pattern or colors which resemble a Scotch plaid;
   checkered or marked with bars or stripes at right angles to
   one another; as, plaid muslin.
   [1913 Webster]

WordNet (r) 3.0 (2006):

plaid
    n 1: a cloth having a crisscross design [syn: tartan, plaid]

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "plaid":
   check, checked, checker, checkerboard, checkered, checkerwork,
   chessboard, colors in patches, crazy-work, harlequin, marquetry,
   mosaic, parquet, parquetry, patchwork, tartan, tessellate,
   tessellated, tessellation, tesserae, variegated pattern